\section{Ultra-High Vacuum Resistance Design}
\label{sec:vacuum}
Taking into account the ultra-high vacuum (10$^{-5}$Pa) \cite{ITEROrg.2004}, the
Vacuum Vessel (VV) must be maintained clean at all costs. For instance all
outgassing materials must be avoided. Usually materials subject to oxidation and
organic materials are outgassing. All of the vacuum facing components have to be
certified. A list of vacuum-agreed components and materials can be found in
the ITER Vacuum Handbook \cite{L.Worth2009}. The structural material for the
ITER in-vessel components is the 316L(N)-IG stainless steel \cite{Aiello2011}.
Consequently, components in front of vacuum must be used under non standard
lubrication, such as molybdenum disulfide (MoS$_{2}$), Diamond-like carbon
(DLC), Tungsten diselenide (WSe$_{2}$), Tungsten sulfide (WS$_{2}$), and
Molybdenum diselenide (MoSe$_{2}$) \cite{ogawa2008research}. In
\cite{winer1967molybdenum} a complete review on the behavior of MoS$_{2}$ can be
found. Besides MoS$_{2}$ has been tested under ultra-high vacuum and has been
found to give an ultra-low friction coefficient \cite{cizaire2002mechanisms}.
This technique has been used extensively for tokamak diagnostic
\cite{ogawa2008research}.
The purpose of baking the robot (normally at 240\textcelsius) is essentially to
free every part from little outgassing pockets of air, situated in the
imperfections of the assembly, and remove any trace of hydrogen and oxygen that
remains in the materials pores. When the robot is not bankable in a reasonable
time at some points then solutions to avoid these pores must be investigated.
The use of non-certified ultra-vacuum components must be avoided. In exceptionalcases these must be confined in a sealed space.
Sealing solutions exist, such as the one used in the AIA \cite{Keller2009} with
Helicoflex\textregistered\ and metal seals. Continuous rotary movement
transmission through this tight box is impossible in the magnetic environment,
so any movement to be transmitted through the box has to be converted into
linear movement and transmitted through flexible leak-tight bellows.